Stable aqueous formulation and use

ABSTRACT

Stable aqueous dispersions of 1,2-benzisothiazolin-3-one containing Xanthan gum which is substantially free from organic solvents.

This application claims benefit of international applicationPCT/GB94/01155, filed May 27, 1994.

The present invention relates to a stable aqueous formulation of1,2-benzisothiazolin-3-one which is substantially free from organicsolvents. 1,2-benzisothiazolin-3-one (hereinafter referred to as BIT) isan established industrial biocide and is particularly effective inprotecting aqueous media against microbiological spoilage. It isparticularly effective as a bactericide and is especially suited to theprotection of latices. Thus, one of its major uses is as an in canpreservative for the preservation of acrylic and acrylate paintemulsions.

BIT has low aqueous solubility and can cause sensitisation in someindividuals. Consequently, for ease of handling and to reduce the riskof sensitisation,.BIT is generally formulated as a liquid composition.These compositions include stable solutions of BIT in an amine solventas disclosed in UK 1,191,253 and UK 1,330,531, and also solutions of BITin propylene glycol or diethylene glycol. None of these compositions areideal and have never been wholly successful in commercial terms.

Because of the low aqueous solubility of BIT, an alternative approach tostable aqueous solutions has been to form the alkali metal salt of BIT,especially the sodium or lithium salt. The lithium salt of BIT isdisclosed in U.S. Pat. No. 4,871,754 as having an aqueous solubilityabove 16% by weight which is approximately twice that of the sodium saltof BIT. The lithium salt of BIT is, however, more expensive tomanufacture. Other approaches to increasing the aqueous solubility ofthe sodium salt of BIT is to include co-solvents such as dipropyleneglycol as disclosed in U.S. Pat. No. 4,188,376 or urea as disclosed inU.S. Pat. No. 4,751,311. The formulation containing dipropylene glycolhas become extremely important commercially. However, formulationscontaining the alkali metal salt of BIT exhibit relatively high pH whichis a disadvantage in some applications. Furthermore, there is an everyincreasing demand for formulations containing a low concentration ofvolatile organic compounds (hereafter VOC). Hence, formulationscontaining solvents such as glycols are becoming less acceptable.

Because of its low aqueous solubility, BIT has also been formulated as adispersion wherein the BIT is generally dispersed by means of anon-ionic or anionic surface active agent. Aqueous dispersionscontaining 33% by weight BIT are known, but tend to be unstable understorage at elevated temperature resulting in gels which are difficult tohandle. Improved dispersions have been attempted by incorporatingsolvents such as propylene glycol, but such dispersions tend to layerand separate on storage at elevated temperature and the BIT consequentlyaggregates and forms lumps. Alternatively, mineral clays have beenincluded in the formulation to structure the aqueous phase with theobjection of inhibiting separation. This tends to reduce aggregation ofthe BIT but the formulation again separates on storage at elevatedtemperature. As with the formulations containing solvents, theformulations containing clays require rehomogenising prior to use andthis can be difficult and expensive. Failure to ensure uniformdistributon of the BIT throughout the formulation can given rise toerrors in metering and subsequent reduction, or even loss, of microbialprotection in the media to be preserved.

Xanthan gum is a polysaccharide thickener which forms a structurednetwork of entangled molecules in water which aids the suspension ofparticulate matter, and can reduce sedimentation. Aqueous dispersionscontaining 33% by weight BIT have been prepared containing both aXanthan gum and propylene glycol as a cosolvent. Again such formulationstend to separate and the BIT tends to aggregate and form lumps. Theaddition of glycols such as mono- and dipropylene glycol to aqueousformulations generally increases the viscosity of the formulation asdisclosed in Examples 16 to 21 of UK Patent Application No. 9300936.3.which relates to aqueous solutions of the sodium salt of BIT. The moredipropylene glycol is added, the higher the viscosity. We have nowsurprisingly found that when the propylene glycol is removed fromaqueous formulations containing BIT dispersed in the presence of Xanthangum and the glycol, the viscosity actually increases. This observationis contrary to the effect of the glycol observed in the formulationsdisclosed in UK Patent Application No. 9300936.3. As a result, we havenow found it possible to make a stable aqueous formulation of BIT in thepresence of a Xanthan gum which is substantially free from organicsolvents.

According to the present invention there is provided a stable aqueousformulation comprising from 5 to 30% by weight microbiologically activeagent containing at least 50% BIT, from 1 to 4% dispersant and from 0.1to 0.5% by weight Xanthan gum, which formulation is substantially freefrom organic solvents.

The remainder of the aqueous formulation is preferably wholly water butmay comprise other materials, such as inorganic salts. The formulation,is however, preferably, completely free from organic solvents.

Preferably, the amount of microbiologically active agent in theformulation is at least 7% and more preferably at least 9%, for example10% by weight of the formulation. The amount of microbiologically activeagent is also preferably less than 25% and more preferably less than22%, for example 20% by weight of the formulation.

The dispersant is preferably either non-ionic or anionic, or a mixturethereof, but is especially anionic. Examples of suitable nonionicdispersants are condensates of ethylene oxide or propylene oxideincluding block co-polymers of ethylene oxide and propylene oxide.Examples of suitable anionic agents are sodium lignin sulphonate and thesodium salt of naphthalene sulphonic acid/formaldehyde condensates.Mixtures of anionic and non-ionic dispersants may also be used.Preferably the amount of dispersant is at least 1.2% and especially atleast 1.5% by weight relative to the total formulation. It is preferredthat the amount of dispersant is less than 3.5%, for example 3% byweight of the total formulation.

The amount of dispersant is commonly calculated on the amount ofmicrobiologically active agent in the formulation. Thus, the ratio ofdispersant to microbiologically active agent is preferably 1:20, morepreferably 2:20 and especially 3:20.

The amount of Xanthan gum is preferably at least 0.2%, more preferablyat least 0.3% and especially at least 0.35% relative to the total weightof the formulation. It is also preferred that the amount of Xanthan gumis less than 0.45%, for example 0.4 relative to the total weight of thecomposition.

The pH of the formulation may be from 1 to 8 but is preferably above 3,more preferably above 5 and especially above 6. It is especiallypreferred that the pH is about neutral, i.e. from 6.5 to 7.5 but morepreferably is not greater than 7.

As noted hereinbefore, the exclusion of the glycol solvent fromformulations of BIT containing a dispersant, Xanthan gum and propyleneglycol results in an increase in viscosity. The formulations of thepresent invention preferably exhibit a viscosity of at least 1000,preferably at least 1200 and especially at least 1500 centipoise asmeasured by Brookfield viscometer using Spindle No. 2 and rotationalspeed of 10 rpm. The viscosity is preferably less than 3,500 andespecially less than 3000 centipoise.

We have obtained a particularly stable dispersion containing about 20%by weight.BIT, about 3% anionic dispersant and 0.3 to 0.4% Xanthan gumrelative to the total weight of the formulation, the remainder beingwater.

The formulation preferably contains BIT as the only microbiologicallyactive agent, but it may contain one or more further microbiologicallyactive agent. The further microbiologically active agent is preferablyone which extends the spectrum of activity against micro-organisms. Whenthe composition contains more than one microbiologically active agent,the further microbiologically active agent or agents may be added to theformulation simultaneously with the BIT or may be added sequentially.

The further microbiologically active agent may be present in up to thesame amount as BIT, but is more preferably less. Thus, according to afurther aspect of the invention there is provided an aqueous formulationcomprising from 2.5% to 15% by weight BIT, from 2.5% to 15% by weight ofa further microbiologically active agent, from 1 to 4% dispersant andfrom 0.1 to 0.5% by weight Xanthan gum. Preferably, the furthermicrobiologically active agent is equal to the amount of BIT, and isespecially about two thirds the amount of BIT.

Preferably the further microbiological agent is the 2:1 Zinc complex of2-mercaptopyridine-1-oxide.

We have obtained a further especially stable dispersion where themicrobiologically active agent comprises 12.3% BIT and 7.7% of the 2:1zinc complex of 2-mercaptopyridine-1-oxide.

The formulation containing BIT may be made from an aqueous paste of BITsuch as that resulting from the filtration of BIT in a filter press(hereafter referred to as ‘press paste’) which is a filter cakecontaining BIT itself and water with substantially no other adjutants.The press paste typically contains from 70 to 80% BIT and has a pH ofbetween 6 and 7.2. Typically, the dispersant is dissolved in water andthe BIT press paste added over a period of time, generally 1-2 hours at20-25° C. in a high shear mixer to disperse the BIT throughout theaqueous phase. Any foaming can be controlled in conventional manner byadding a suitable anti-foam agent which is compatible with othercomponents of the formulation and which does not interfere with themicrobiological activity of the BIT or the further microbiologicallyactive agent, if present. The suspension is then milled until thedispersed matter has a particle size below 20 μ, preferably below 10μand especially below 5μ. The remainder of the water is then addedfollowed by the Xanthan gum, which is preferably added as a solid.Addition of the Xanthan gum is made under rapid agitation in order toprevent the formulation of lumps.

The formulations of the present invention are microbiologically activeand can be used as industrial biocides to protect media againstmicrobiological deterioration.

The invention is further illustrated by the following non-limitingexamples wherein all references to amounts are in parts by weight unlessstated to the contrary.

EXAMPLE 1

Sodium lignin sulphonate (30 parts) is dissolved in water (350 parts) bystirring at 20-25° C. BIT press paste (266 parts containing 75% BIT and25% water) was added over 30 minutes and uniformly distributed in aSilverson mixer. The resulting dispersion was then milled in ahorizontal bead mill in the presence of Ballotini beads until amean-particle size of 95% BIT below 5μ was obtained. The remainder ofthe water was then added followed by Xanthan gum (4 parts) which wasadded as a dry powder over 30 minutes. The formulation obtainedcontained 20% BIT, 3% dispersant and 0.4% Xanthan gum, the remainderbeing water. It had a viscosity of about 3100 centipoise as measured byBrookfield viscometer using Spindle No 2 and a rotation speed of 10 rpm.

The formulation exhibited excellent stability when stored at 40° C. for1 month with only about 1% separation.

Comparative Example A

Example 1 was repeated except that 10% of the water was replaced by anequivalent amount of dipropylene glycol. The viscosity of the resultantformulation was reduced to about 1650 centipoise. This is contrary tothe effect of adding dipropylene glycol to aqueous solutions of thesodium salt of BIT as disclosed in UK Application No. 930936.3.

EXAMPLE 2

Example 1 was repeated except that the amount of sodium ligninsulphonate and the amount of BIT press paste was reduced by 50% withconsequential adjustment to the amount of water. The resultingformulation contained 10% PIT, 1.5% dispersant and 0.4% Xanthan gum, theremainder being water. When stored for 1 month at 40° C., theformulation showed no separation.

EXAMPLE 3

Sodium lignin sulphonate (15 parts) was dispersed in water (200 parts)by stirring at 20-25° C. BIT press paste (168 parts containing 73.6%BIT) was added and dispersed uniformly throughout the aqueous phase bymixing for 10 minutes in a Silverson mixer.

The course dispersion so obtained was then milled in a horizontal beadmill in-the presence of Ballotini beads and with a peripheral disc speedof about 1500 ft/min. Milling was continued until the mean particle sizeof the BIT has a distribution of 95% below 5 μ.

An aqueous dispersion of the 2:1 zinc complex of2-mercaptopyridine-1-oxide (160 parts; ex Olin Chem. Co., USA) togetherwith the remaining water was added and thoroughly mixed.

Xanthan gum (5 parts) was then added rapidly under high shear mixing andmixing continued for a further 20 minutes. The resulting formulationcontained 12.3% BIT, 2.0% dispersant, 7.7% 2:1 zinc complex of2-mercaptopyridine-1-oxide and 0.5% Xanthan gum, the remainder beingwater. The formulation exhibited excellent stability when stored for 1month at 40° C. with no observable phase separation.

What is claimed is:
 1. A stable aqueous formulation comprising from 5 to30% by weight of a microbiologically active agent containing at least50% BIT, from 1 to 4 dispersant and from 0.1 to 0.5% by weight Xanthangum, which formulation is substantially free from organic solvents.
 2. Aformulation as claimed in claim 1 wherein the remainder of theformulation is water.
 3. A formulation as claimed in either claim 1 orclaim 2 wherein the amount of microbiological agent is at least 9% byweight of the total formulation.
 4. A formulation as claimed in claim 1wherein the amount of microbiological agent is less than 22% by weightof the total formulation.
 5. A formulation as claimed in claim 1 whichcontains only BIT as the sole microbiologically active agent.
 6. Aformulation as claimed in claim 1 which has a viscosity of at least 1000centipoise.
 7. A formulation as claimed in claim 1 which has a viscosityof less than 3500 centipoise.
 8. A formulation as claimed in claim 1which comprises about 20% BIT, about 3% dispersant and about 0.4%Xanthan gum by weight of the total formulation.
 9. A formulation asclaimed in claim 1 which comprises about 10% BIT, about 1.5% dispersantand about 0.4% Xanthan gum by weight of the total formulation.
 10. Aformulation as claimed in claim 1 which comprises about 12.3% BIT, about2.0% dispersant, about 7.7% of the 2:1 zinc complex of2-mercaptopyridine-1-oxide and about 0.4% Xanthan gum.
 11. A formulationas claimed in claim 1 wherein the dispersant is anionic.
 12. Aformulation as claimed in claim 11 wherein the dispersant is sodiumlignin sulphonate.
 13. A formulation as claimed in claim 1 wherein thedispersant is a condensate of ethylene oxide or propylene oxide or ablock co-polymer of ethylene oxide or propylene oxide.
 14. A formulationas claimed in claim 1 wherein the microbiologically active agent ismilled in the presence of the dispersant.